The invention relates to electric machines, in particular asynchronous machines. In particular, the present invention relates to measures for increasing the performance of asynchronous machines at maximum rotational speed.
Electric machines generally comprise a stator and a rotor. In the case of asynchronous machines, the stator comprises a stator winding by means of which a circumferential stator magnetic field is produced. The circumferential stator magnetic field moves at a rotational speed that corresponds to the quotients of the basic frequency of the controlling driving system and the number of pole pairs of the electric machine.
In contrast to synchronous machines, the rotors of asynchronous machines comprise short-circuited conductors. As soon as the mechanical rotational speed of the rotor deviates from the synchronous rotational speed, the magnetic flux through the short-circuited conductors changes so that currents are induced therein. The induced currents produce an exciter magnetic field that interacts with the stator magnetic field and as a result produces a torque.
In contrast to synchronous machines, asynchronous machines have the advantage that permanent magnets not required. Therefore, due to the high level of reliability and the high degree of efficiency, said asynchronous machines are frequently used in the field that is relevant to the driving cycle as traction motors for motor vehicles driven by an electric motor, traction machines and similar vehicles.
The publication U.S. Pat. No. 8,154,167 B2 describes a highly efficient asynchronous machine having a high torque density and a high maximum rotational speed, wherein the stator yoke thickness is between 30% and 70% greater than the stator tooth length and the rotor yoke thickness is between 30% and 70% greater than the rotor tooth length.
The publication EP 2 202 871 A2 discloses an electric machine having a stator, wherein the ratio of a stator yoke thickness with respect to a stator tooth width can be at least 5:1 and the ratio of a rotor yoke thickness with respect to a rotor tooth width can be at least 5:1.
Because of the function they perform, asynchronous machines require a good electromagnetic coupling between the stator and the rotor. The proportion of the magnetic flux that is not interlinked by means of the stator and the rotor is referred to as the flux leakage and reduces the achievable torque of the electric machine. The reduction of the achievable torque is particularly high if the electric machine is operated at a voltage limit and at a maximum torque. This occurs in particular in the case of traction motors at high motor rotational speeds. The leakage of the magnetic flux then reduces the achievable torque at high rotational speeds so that high vehicle velocities cannot be maintained in the case for example of a strong wind resistance or on gradients, or acceleration processes are prolonged.